The invention relates to a plastics container for food. It more particularly relates to a mass produced thermoplastics container of the type used for storage at shelf temperature (i.e. non-refrigerated storage) of foods requiring heat processing. In particular steam retort sterilization as part of the packaging and subsequent processing and which may be used ultimately after directly heating the contents in the container in a microwave oven.
The invention provides the public a useful choice in selecting foods in convenience packaging wherein the package can provide the cooking vessel and even the eating receptacle for the consumer. It provides a disposable hermetically sealed container package which can be heated in a microwave oven and, if required, eaten hot direct from the container and moreover, if to be only partially consumed, the container may be tightly reclosed repeatedly after being opened, in which state it may be preserved for some time under refrigeration.
The container of the invention is formed from thermoplastics barrier sheet, that is, an extruded multilayer sheet that contains one or more layers highly resistant to the permeation of at least oxygen and water vapor despite the overall thinness of the sheet used for forming the container.
Where low acid foods are to be preserved in a shelf stable package using a container of the type of the invention, heating in a steam pressurized retort is required to cook the food during the course of a heat processing cycle that ensures a temperature high enough (e.g. up to 130.degree. C.) and a time long enough to ensure that "commercial sterility" of the contents results. The package must remain hermetically sealed during and following the heat process to ensure food spoilage organisms do not enter the package. The package must remain sealed hermetically until the time it is deliberately opened for consumption, and must remain effectively sterilized of pathogenic food spoilage organisms regardless of whether the food is served heated or not.
During heat processing in a retort, even of the balanced pressure type, it is virtually inevitable that some differential pressure will occur between the pressurized interior of the retort and the package interior. The inevitable pressure induced stresses on a container in such an environment when accompanied by at least the degree of temperature induced softening unavoidable in the case of a thermoplastics container is not conducive to ensuring that a container of the general type would not leak, especially considering the relatively thin (typically 200 to 1200 micron) thermoformable barrier sheet used for the body and cover of the package. For this reason the container of the invention discloses an effective while relatively economical clamp ring to enable the package as a whole to resist the stresses imposed while heat processing and, at the same time, providing tamper evidence, ease of opening, microwave oven heating possibility (i.e. non-metallic construction) and repeated resealability all in an economical throw-away package having utility at all phases of the production, distribution, preparation and serving phases.